1. Field of the Invention
The invention relates to a temperature measurement device for a power semiconductor device.
2. Description of the Related Art
In general, in a power converter that uses a power semiconductor device such as an IGBT (Insulated Gate Bipolar Transistor), a power semiconductor device and an integrated circuit (IC) that drives and protects the power semiconductor device are formed on different chips. This is because the manufacturing cost of forming them on different chips is lower than the cost of forming them on the same chip.
Furthermore, such power converters are necessarily provided with a device for measuring a temperature of the power semiconductor device, since the temperature of the power semiconductor device has to be detected to protect the power semiconductor device from overheating. Since, as mentioned above, the power semiconductor device and the integrated circuit are constituted on separate chips, as shown in FIG. 20, a temperature sensor 1 is disposed on a chip 2 of the power semiconductor device, whereas a detection circuit 3 for controlling the temperature sensor 1 and processing an output signal is disposed on a chip 4 of the integrated circuit.
One type of temperature sensor is known from JP-A-3-148861 (corresponding to U.S. Pat. No. 5,070,322) in which a diode (hereinafter referred to as a diffusion diode) is formed by means of diffusion on a surface of a silicon substrate of a power semiconductor device, and the temperature dependency of a reverse leakage current of the diode is utilized.
Furthermore, a type of temperature sensor that makes use of the temperature dependency of a forward voltage of a diffusion diode is also known in the art. See JP-A-5-129598 (corresponding to U.S. Pat. Nos. 5,355,123 and 5,349,336) and JP-A-8-316471 (corresponding to U.S. Pat. No. 5,736,769). In a temperature sensor according to these configurations, since a PN junction is stably formed in a single crystal substrate of silicon, in comparison with a diode formed from polysilicon, the linearity of the temperature dependency of the forward voltage is excellent.
However, in a temperature measurement device that uses an existing diffusion diode as a temperature sensor, when a diffusion diode is formed on a silicon substrate of a power semiconductor device and a forward voltage of the diffusion diode is utilized per se, sufficiently high temperature detection accuracy can be obtained only with difficulty. The reason why is that current flowing to the diffusion diode fluctuates and a PN junction of the diffusion diode is manufactured with dispersion.
In addition to the above, since a temperature sensor and a detection circuit are disposed on separate chips, there is no correlation between the fluctuation of the temperature sensor and the dispersion of the detection circuit. Accordingly, in a temperature measurement device in which these are combined, the respective dispersions are effectively added. In this configuration, a detection voltage of the detection circuit is controlled in accordance with a forward voltage of the IGBT to reduce the dispersion of the temperature measurement device. However, as adjustment is troublesome and time consuming, there is a problem in that this leads to increased cost.
Furthermore, when an IGBT is turned off, holes (positive carriers) piled up in an N-type substrate that becomes a drift region when the IGBT is turned on, though drawn by an emitter, leak into the diffusion diode. Thereby, a parasitic thyristor operates made of an NPN transistor constituted of an N-type region of the diffusion diode, a P-well of the diffusion diode and an N-type substrate and a PNP transistor constituted of a P-well of the diffusion diode, the N-type substrate and a P-type region that becomes a collector region of the IGBT. Accordingly, there is a problem in that the diffusion diode may be damaged. Alternatively, there is a problem in that a parasitic current flows in the diffusion diode and thereby the detection circuit connected to the diffusion diode may be damaged.
Still further, in order to always supervise a temperature of a power semiconductor device to optimally control the semiconductor device, the necessity arises of constantly outputting from the temperature measurement device a voltage proportionate to the temperature of the power semiconductor device. However, in between when the IGBT is in an ON-state and when it is in an OFF-state, a current flowing from an anode of the diffusion diode varies. Accordingly, it is difficult to constantly obtain a forward voltage proportionate to the temperature.
The reason why, when a constant current flows to the anode of the diffusion diode, the current flowing in the diffusion diode varies depending on a state of the IGBT, is as follows. When the IGBT is turned on, a potential of a region in the proximity of the diffusion diode of the N-type substrate is reduced, and thereby a junction of a P-well of the diffusion diode and the N-type substrate is forward biased. Accordingly, a current flowing from the anode of the diffusion diode to a cathode thereof partially leaks and flows through the P-well of the diffusion diode and the N-type substrate to an emitter of the IGBT. Conversely, when the IGBT is turned off, the junction of the P-well of the diffusion diode and the N-type substrate is reverse biased. Accordingly, the current flowing in the diffusion diode does not leak toward the IGBT.